Enhancement and Control of Optical Third-harmonic Generation and Fluorescence from Dielectric-metal Core-shell Microstructures
Author:Li Zhi Qin
Supervisor:wang zhen lin
Plenty researches on surface plasmon resonances have been performed in the past decades.When metallic nanostructures are illuminated by incident electromagnetic field,nearfield enhancement would be generated around the metal surface,which benefits the efficiency of various kinds of light-matter interactions.Meanwhile,the generated localized surface plasmon resonances(LSPR)are extremely sensitive to the geometric parameters of the metallic nanostructures and the surrounding environment.These characteristics enable the further applications in bio-sensors,optical detection,optical antennas,all optical control and so on.In this thesis,we will give detailed studies on enhancing and controlling several light-matter interactions in dielectric-metal core-shell microstructures.The thesis is mainly composed of four sections which are arranged as following:(1)The third harmonic generation(THG)from two kinds of gold hemispherical shell arrays was studied.Highly incoherent THG signals were obtained,suggesting that the short lifetime SPs may involve in the nonlinear conversion as quasi-particles.The hemispherical shell arrays are prepared by physically depositing a thin Au layer on a colloidal crystal template,followed by etching the silica template in hydrogen fluoride acid solution.On the purpose of comparison,the orientation of the shells is controlled so that two kinds of SP structures of similar geometric characters were fabricated with their opening arranged towards or backwards to the substrate.In the nonlinear experiments,the far field behavior of THG signals highly depends on the orientation of the hemisphere.When the opening of the hemisphere is parallel with the wavevector of the fundamental laser,the far field THG signals are highly collimated into diffraction orders;while the far field THG signal from the anti-parallel sample is diffused and depolarized.The measured and calculated linear spectra show that an enhanced transmission occurs in the near-infrared due to the excitation of SP resonant modes.However,the dominant mode in each orientation is quite different.A high quality mode with less radiation damping is excited efficiently in the parallel configuration while a low quality cavity mode with heavy radiation damping is dominant in the anti-parallel configuration.As the shell array is highly ordered,the diffused and depolarized THG signals suggest that the nonlinear process in anti-parallel configuration may not rise from the nonlinear conversion of the coherent photons.Thus we propose that the SPs may directly involve in the nonlinear process as quasi-particles in the anti-parallel configuration,and the strong damping nature of the SP mode leads to the incoherent THG radiation.And in SP resonant conditions,a larger mode volume may result in a stronger radiation damping,along with a shorter SP lifetime.(2)THG signals from completed dielectric-metal core-shell nanospheres array is both theoretically predicted and experimentally measured,revealing that the far field behavior of THG signals can be strongly tuned by Fano type resonances.According to calculated and experimental linear spectra,high quality cavity modes can be excited in a complete metallic nanoshell.The narrow cavity modes can interfere with the broad sphere modes,leading to even narrower fano type resonances.When the wavelength of the fundamental laser varies around the fano resonances,the local field of the nanostructure may engage significant variation,leading to the alternation of the corresponding far field pattern of generated THG signal.While the metallic spheres are arranged into ordered arrays,the tuning effect reduces into the control of the far field diffraction orders.The nonlinear numerical calculations show that when the electric quadrupolar mode is excited by linear polarized fundamental wave,the intensity of the first order diffraction patterns in the k-E plane is much more stronger that the others.And if the electric octupolar mode or the magnetic dipole mode is excited,the zero order diffraction pattern becomes the most intense.The metallic nanospheres array is prepared by deposing gold on both sides of the free-standing dielectric spheres array.THG diffraction orders are achieved in the far field measurements,and the radiation behavior matches well with the theoretical predication.(3)Theoretical analysis on double resonance condition in metallic core-shell nanostructure is performed in order to optimize the efficiency of THG while the core material has a high third order nonlinear susceptibility and the nonlinear susceptibility in metal shell is neglectable.As the core-shell nanostructure supports tunable high quality cavity modes ranging from visible area to near infrared with various mode symmetries,the double resonance condition can be realized while both the fundamental wave and the THG signal excite a multipolar cavity mode.The nonlinear numerical calculation shows that when the double resonant condition is achieved,the THG efficiency is dramatically enhanced in comparison with the condition that only one cavity mode is excited.As plenty modes are supported by the nanocavity,various kinds of double resonance condition can be formed by matching two different multipolar cavity modes.The THG efficiency differs up to three magnitudes under different double resonance condition.Further investments show that the THG efficiency can be optimized when the two multipolar cavity modes involved in the double resonance condition have the same angular momentum.(4)Tuning effect of the fluorescence in dye-doped core-shell nanospheres array is experimentally measured.The existence of the core-shell structure leads to a spectral reshaping of the fluorescence with sharp peaks corresponding to the resonant wavelengths of the multipolar cavity modes.Spectral and directional measurements were performed focusing on three samples with different core diameter.Experimental results show that the far field distribution of fluorescence is also strongly affected by the cavity modes,resulting in various directional radiation.This phenomenon indicates that the fluorescence signals from dye molecules may radiate into far field via coupling to the plasmonic modes.As the inherent radiation behavior differs a lot between electric multipolar modes and magnetic multipolar modes,the fluorescence signals with fixed polarization state may engage distinct directional behavior when coupled to different cavity modes.The electric multipolar cavity modes would reshape the far field fluorescence in the k-E plane,while the magnetic multipolar cavity modes would reshape the far field fluorescence in the k-H plane.Thus the degenerated electric modes and magnetic modes can be distinguished by analyzing the polarization state of the laterally emitted fluorescence.